a(n) is the number of squarefree divisors, but the set of unitary divisors of n is not the set of squarefree divisors, e.g., set of unitary divisors of number 20: {1, 4, 5, 20}, set of squarefree divisors of number 20: {1, 2, 5, 10}. - Jaroslav Krizek, May 04 2009